Hi Normal Distribution: μ = 104 and σ = 17 percentage of IQ scores are between 87 and 121? Each 17 from mean: between z = -1 and z = 1 one standard deviation from the mean accounts for about 68.2% of the set***** two standard deviations from the mean account for about 95.4% and three standard deviations from the mean account for about 99.7%. Important to Understand z -values as they relate to the Standard Normal curve:Wish You the Best in your Studies.